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Beware your Nitrate test

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I was running low on my nitrate test chemicals so I bought a new test kit. Both old and new are API test kits. In fact the manufacturing lot number is the same on both.

The picture shows an obvious difference in test results between the old test kit and the new. The test tube marked "old" is from the almost empty test kit. The test tube marked "new" is from the fresh just opened test kit.

I assume I should believe the fresh test kit but I guess I should buy a third kit and do two out of three.

I can only think that the test chemical from the bottle that says "IMPORTANT! shake for 30 seconds" changes concentration if not enough shaking is done. More dense chemical settles to the bottom and as the bottle is depleted, the solution ends up getting more and more concentrated.

Do they still make the AP dry tab test kits. I always like those better than the liquid ones. I just have not been able to find them. I may just buy a HACH kit through work. Just need to find what range I want.

The 2 new test kits agree with each other (about 10ppm). The old nearly empty test kit reads high at about 20ppm, a 100% error.

I guess its just something to be aware of, shake well, and I know I'm going to replace the chemicals long before they've nearly run out.

-dickm

They will go bad eventually just sitting on the shelf for a year in the store. So if you can use new tests, compare them with the lfs results of the same water sample to test it. There really not many ways to test nitrite i can think of...so buy the master kits and test the ammonia and nitrate tests. Ammonia by adding it to dechlored water, nitrate from ur tank.

I dont think its an issue of going bad. I think its an issue of getting more concentrated without adequate shaking.

If you look at the solution when you are adding drops to the test tube, the drops sink right to the bottom of the tube. They are much denser than water. It must do the same in the bottle and thus it will tend to get stronger as you get to the bottom of the bottle without adequate mixing/shakng.

So as you use up the bottle, the concentration changes and it appears you have more nitrate than you really do.